Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 79

4.3 Examples of Hydrothermal Deposits and Ore-Forming
Processes
We have selected five types of deposit to illustrate how different types of fluid in
diverse geological settings can lead to the formation of an ore body. The list is by no
means exhaustive - in a short text such as this it is impossible to describe the
vast range deposits that form as the result of circulation of hydrothermal fluids.
However our selection will, we hope, suffice to illustrate the essential features of
this class of deposits. As with the description of magmatic deposits the emphasis is
not on the characteristics of the deposits themselves but more on the processes that
produced them.
4.3.1 Volcanogenic Massive Sulfide (VMS) Deposits
We start with this type of deposit because they are among the best understood of all
ore deposits. There are various reasons for this: the ore bodies are relatively simple,
both in their structure and their composition and mineralogy, and they have also
been studied intensively over the last decades. But more to the point is the fact
that they are one of very few deposits whose formation, by way of precipitation of
sulfides at or just below the ocean floor, we can observe directly (other examples
of active ore formation include the accumulation of heavy minerals in placer
deposits and the accumulation of sulfidic sediments in sedimentary basins. These
are described in Chap. 5).
The discovery in 1977 by scientists in the Alvin submersible of active hydro-
thermal vents - black smokers - on the ocean floor is one of the most important
advances in earth (and biological) sciences of the past decades. The discovery has
had profound implications for the origin and evolution of the oceanic crust and for
the biological sciences, and it also opened a window through which we can study, in
real time, the processes that generate an ore body. At each hydrothermal vent,
sulfides rich in Zn, Cu and Pb precipitate in the chimneys that build up around each
upwelling jet of hydrothermal fluid. The same sulfides separate out from the
hydrothermal plume and settle onto the ocean floor. Most accumulations of sulfide
minerals on the modern sea floor are relatively small but the long-lived system that
built the TAG mound on the Mid-Atlantic ridge is estimated to contain about three
million tons of sulfide grading 2% Cu with smaller concentrations of Zn and Au.
If such a deposit were present on land (and not in a region hostile to mining), it
certainly would be exploited.
VMS deposits were among the first ever to be mined: ores on Cyprus and in
Spain, for example, were exploited over 2000 years ago and provided much of the
copper used in the bronze weapons of Roman centurions. In the early part of the last
century, when the opinions of American geologists like Lindgren held sway, these
deposits were interpreted as epithermal replacement bodies produced by the